1. Field of the Invention
The present invention relates to an image printing apparatus which prints a color image by superimposing a plurality of images of various colors and a color misregistration correction method in the apparatus.
2. Description of the Prior Art
There have been available image printing apparatuses such as color copying machines and color printers which print and output color images by superimposing yellow (Y), magenta (M), cyan (C), and black (K) images.
For example, in a tandem color copying machine using an electrophotographic process, image printing units, each including a photosensitive drum, charging device, scanning optical device, developing device, and the like, are prepared for the respective colors and arranged along an intermediate transfer belt in the form of an endless belt. This copying machine is designed to print a color image by superimposing images of the respective colors on the rotating intermediate transfer belt. The color image printed on the intermediate transfer belt is transferred onto a transfer sheet and output.
When a color image is to be printed by superimposing images of the respective colors in this manner, color misregistration occurs to result in a failure to obtain a beautiful image unless the printing positions of the images of the respectively color accurately match. Therefore, test images for color misregistration correction called registration marks are printed on the intermediate transfer belt. These marks are read by optical sensors to obtain necessary correction amounts, and the printing positions of images are corrected.
The correction amount obtained by printing and measuring registration marks includes a small correction amount less than the pixel unit. Of such corrections, correction on a pixel basis in the main scanning direction is realized by adjusting the timing of supplying an image signal to the laser diode of each scanning optical device on a clock basis. Correction with an amount less than the pixel unit in the main scanning direction is performed by, for example, surface phase control on each polygon mirror which scans a laser beam on a photosensitive body in the direction of width.
In color misregistration correction, a necessary correction amount is derived by printing and measuring registration marks, and both correction on a pixel basis and correction with an amount less than the pixel unit are executed in accordance with the correction amount. This process is repeated until the color misregistration amount is made to fall within an allowable range.
Correction on a pixel basis is finished in a short period of time because it can be done by only adjusting the timing of supplying an image signal to each laser diode on a clock basis. On the other hand, in surface phase control on each polygon mirror which is performed to make correction with an amount less than the pixel unit, a long period of time (several seconds) is required between the instant at which phase adjustment is made and the instant at which the rotation of each polygon mirror stabilizes. Conventionally, both correction with an amount less than the pixel unit, which requires a long period of time, and correction on a pixel basis, which can be finished within a short period of time, are executed for every correction based on the printing/measurement of registration marks. As a consequence, the cycle between the time given registration marks are printed and the time the next registration marks are printed is prolonged. It therefore takes a long time to complete a series of correcting operations.